The present invention relates to the suppression of gas turbine engine noise, and more particularly to an exhaust flow guide for aero-engine exhaust jet noise reduction.
Noise has been a significant negative factor associated with the commercial airline industry since the introduction of the aircraft gas turbine engine. Considerable effort has been directed toward quieting aircraft engines.
Aero-engine exhaust jet noise is a dominant noise source of aircraft gas turbine engines at high power settings, for example, during a flight take-off operation. Jet noise is not generated within the gas turbine engine, but is caused by turbulence resulting from large velocity gradients produced by viscous shearing of rapidly moving gases as they are exhausted into the relatively quiescent surrounding atmosphere at the boundary between the exhaust gases and the atmosphere. Since the acoustic gas power is exponentially related to the velocity of the exhaust gases, that is, proportional to V8, decreasing the velocity of the exhaust gases prior to discharge into the atmosphere substantially reduces the exhaust jet acoustic power.
In comparison with early turbine engines, modern gas turbine engines have reduced jet noise significantly. Many types of modern gas turbine engines are of the mixed flow variety, wherein, a primary fluid stream is mixed with a secondary fluid stream prior to discharge of the exhaust fluid into the outer atmosphere, as a common thrust-producing mixed flow fluid stream. Generally, the primary fluid stream consists of the high velocity, high temperature exhaust gases flowing from the turbine stage of the core engine and the secondary fluid stream consists of air or gas at a lower temperature and velocity, for example, from the engine fan stage through an annular bypass duct surrounding the core engine. As is well known in the art, such a mixed flow has two beneficial effects. First, engine thrust is improved since the mixed gases have a higher mass-velocity product than that of the turbine exhaust gases alone. Secondly, the noise level is reduced due to the mixed gases having a lower velocity than the velocity of the turbine exhaust gases.
Arrangements for mixing the core engine exhaust gases with bypass flow are well known in the art. The prior art mixers are effective in reducing the overall jet noise, nevertheless the prior art mixers are generally used with gas turbine engines having a long cowl nacelle which extends downstream of a core engine exhaust so that the mixing action generally occurs within the nacelle duct at the downstream end section. It is not popular to use the prior art mixer with gas turbine engines having a short cowl nacelle because the core engine extends downstream of the nacelle outlet and the air flow discharged from the bypass duct is mixed with unbounded air before reaching the core engine exhaust end.
The viscous shearing of the rapidly moving exhaust gases, even after being mixed with bypass duct air flow by the mixer, discharged into the relative quiescent surrounding unbounded air, still produces a turbulence region immediately downstream of the exhaust end of the gas turbine engine, effectively, along a longitudinal length of up to twenty times the diameter of the exhaust end of the gas turbine engine. This turbulence region produces the substantial proportion of exhaust jet noise and is called the jet noise contribution volume. Efforts have been made to effect a better mixing of engine exhaust gases in order to reduce the jet noise contribution volume, thereby resulting in exhaust jet noise reduction.
U.S. Pat. No. 4,786,016, issued to Presz, Jr. et al. on Nov. 22, 1988 discloses a casing surrounding a fluid stream over which an unbounded fluid flows in a downstream direction having a plurality of alternating, adjoining troughs and ridges in its external surface, extending in the downstream direction to a thin trailing edge of the casing, which will thereby have a wave-like shape. According to Presz, Jr. et al. this type of casing structure which can be applied to both long cowl nacelle and short cowl nacelle gas turbine engines and to both a nacelle outlet and a core engine exhaust nozzle, is used to prevent or reduce the area of a stream-wise two-dimensional boundary layer separation on the external surface of the casing, and thereby reducing the surface drag. Nevertheless, the wave-like shaped casing structure is similar to the prior art mixers and promotes the mixing of the fluid flow discharged from the casing within the surrounding unbounded air. Thus, the wave-like shaped casing structure will reduce exhaust jet noise as well, when formed as an air end section of a gas turbine engine or the exhaust end of a core engine.
The Applicant has developed a gas turbine exhaust jet noise reduction assembly for a gas turbine engine, which is described in the Applicant""s co-pending U.S. patent application Ser. No. 09/737,599, filed on Dec. 18, 2000. The assembly includes an exhaust shroud having a tubular wall extending between a forward end and an aft end, adapted to be affixed to the gas turbine engine exhaust and for discharging engine exhaust gases without substantial blockage thereto. The assembly has perforations formed in the shroud wall for fluid communication between regions at both sides of the shroud wall, thereby resulting in fluid flow across the shroud wall to enhance mixing of the engine exhaust gases with a surrounding fluid flow.
Another example of efforts toward engine jet noise reduction is described in U.S. Pat. No. 5,491,307, issued to Wlezien on Feb. 13, 1996. Wlezien describes a single expansion ramp extending from the exhaust opening of a fluid nozzle which is capable of exhausting supersonic fluid flow which forms standing shock waves generating noise. The ramp has a face lying adjacent the supersonic fluid flow and holes which pass through the face and entirely through the ramp. The face has a porosity of at least 4% so that compression waves are created in the supersonic fluid flow and the amplitude of the noise is decreased. The expansion ramp however, does not reduce the jet shear noise generated by the shear layer between the jet exhaust and the ambient flow.
It is desirable to develop more effective new and alternative devices for aero-engine exhaust jet noise reduction, particularly for reducing the jet shear noise generated by the shear layer between the jet exhaust and the ambient flow. It is also desirable to have new and alternative devices for aero-engine exhaust jet noise reduction that are simple to manufacture and maintain, and are applicable to different types of gas turbine engines.
It is one object of the present invention to provide a device for effectively suppressing aero-engine exhaust jet shear noise.
It is another object of the present invention to provide a gas engine exhaust jet noise reduction device that is simple to manufacture and maintain.
It is yet another object of the present invention to provide a gas engine exhaust jet noise reduction device applicable to gas turbine engines having either a short cowl nacelle or a long cowl nacelle.
It is a further object of the present invention to provide a device to change jet noise directivity and reduce its power level.
It is a still further object of the present invention to provide a device to enhance mixing of the engine exhaust gases with surrounding fluid flow, thereby reducing the jet noise contribution volume.
It is a still further object of the present invention to provide a method of reducing exhaust jet shear noise by asymmetrically guiding engine exhaust flow to change the noise directivity and reduce its power level.
In general terms the jet noise directivity of a gas turbine engine is changed and the jet noise power levels are reduced by means of an exhaust flow guide made of a curved sheet metal attached to an engine exhaust end to form a partial portion of a nozzle to asymmetrically affect the viscous shearing of rapidly moving exhaust gases into a relatively quiescent surrounding fluid flow at the boundary between the exhaust gases and the surrounding fluid flow.
In accordance with another aspect of the present invention, there is a gas engine exhaust jet noise reduction device provided for a gas turbine engine having an exhaust end. The device comprises an exhaust flow guide adapted to be attached to the exhaust end. The exhaust flow guide has a piece of sheet metal curved to form a partial portion of a nozzle having curved leading and trailing edges, and side edges. The curved leading edge has a circumferential length greater than a circumferential length of the curved trailing edge. A distance from a central point of the curved leading edge to an imaginary straight line extending between two circumferentially opposed extremities of the curved leading edge is greater than one quarter of a diameter of an inner periphery of the exhaust end. Thus, the exhaust flow guide asymmetrically guides an exhaust flow to change a jet noise directivity and reduce jet noise power levels.
It is preferable that a distance from the central point of the curved leading edge of the exhaust flow guide to a center point of the curved trailing edge of the exhaust flow guide is not smaller than 0.3 of the diameter of the inner periphery of the exhaust end. The piece of sheet metal is preferably in a trapezoidal shape or a triangular shape. In the latter case, the length of the trailing edge is actually reduced to a point.
The piece of sheet metal forming the exhaust flow guide, according to one embodiment of the present invention is attached to and extends rearwards from the engine exhaust, and is curved about a central longitudinal axis of the engine to form a partial portion of a nozzle. The center point of the curved leading edge of the exhaust flow guide is positioned circumferentially adjacent to a lowest point of the inner periphery of the exhaust end when the aircraft having the gas turbine engine is grounded. The exhaust flow guide is preferably formed in a shape and attached to the exhaust end, forming a smooth continuation of a nacelle exhaust nozzle of the gas turbine engine. Thus, the exhaust flow guide is preferably formed as a partial portion of a substantially cylindrical nozzle or a partial portion of a truncated conical nozzle, depending on the shape of the nacelle exhaust nozzle to which the exhaust flow guide is attached.
The exhaust flow guide according to another embodiment of the present invention can be attached to both the engine exhaust end and a fan bypass nacelle outlet to asymmetrically affect both the engine exhaust gas flow and the bypass air flow discharged by the fan bypass nacelle outlet.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter.